This invention relates to imaging devices, and particularly to an apparatus and method for processing signals of an imaging device having a most suitable digital signal processing function for a color video camera or the like.
An example of the digital signal processing for a video camera in association with this invention is described in Japanese Patent Publication Gazette No. 63-45153.
The solid-state imaging devices include various different types, and can be roughly divided into the MOS type and the CCD type. In general, the MOS type image sensor has multiple outputs, and the CCD type image sensor has a single output. From the viewpoint of digital signal processing, the CCD type sensor having a single output needs only one A/D converter, and is thus more advantageous than the MOS type sensor which requires a large number of A/D converters. The CCD type sensor includes various different types, and at present the most common one is the picture element mixture read type CCD sensor which is described in The Institute of Television Engineers of Japan, Technical report, TEBS101-1, ED836, pages 1 to 6. This CCD sensor is different in construction from that disclosed in Japanese Patent Publication Gazette No. 63-45153, but it is able to make digital signal processing using the same system.
The digital signal processing in the video camera using the picture element mixture read type CCD sensor has many problems to be solved, one of which is the quantize bit number problem with the A/D converter.
The A/D converter always converts an input voltage within a certain range into a constant digital value. At this time, since the input analog value is not converted into a digital value in one-to-one correspondence, error occurs. This error enters into the signal component as quantize noise.
It is generally known that the quantize bit number, under which the S/N ratio in the video signal processing is not seriously affected by the quantize noise, is 8 bits in the luminance signal and 6 bits in the chrominance signal at the final output stage of the camera.
In the video camera using the above CCD sensor for the digital signal processing, the following processings are performed for producing the luminance signal and chrominance signal, which processing increases the quantize noise.
For the luminance signal:
Gamma processing: The signal is raised to the power of 1/2.2 for the correction of the 2.2-power characteristic of the Braun tube. For this reason, the gain of the luminance signal of low level is increased, thus emphasizing the quantize noise.
For chrominance signals:
Gamma processing: Same as above.
Matrix: The chrominance signal such as RGB is produced by arithmetic processing of the sensor output signals associated with complementary color filters of magenta (Mg), cyan (Cy), yellow (ye) and green (G). At this time, since the chrominance signals of RGB are produced by subtraction processing, the produced RGB signal has a smaller level than the original signal level. Since the signal component is small, the noise component becomes apparently large so that the S/N ratio is decreased by quantize noise.
White balance: The level of the RGB signal produced by the above matrix is changed by the color temperature of the light source. For correcting this, the gains of the R and B signals are increased or decreased. At this time, since the quantize noise of the chrominance signals which are increased in gain is also emphasized at the same gain, the S/N ratio is decreased.
The quantize noise in the above processings is increased particularly for the chrominance signal. At the final output, in order that the quantize noise level included in the chrominance signal is made equal to that in the case of the 6-bit quantization, it is necessary that the quantize bit number of the A/D converter for converting the output signal from the sensor into a digital signal is selected to be at least about 10 bits. However, at present it is difficult to realize the A/D converter of 10 bits or above for video, and thus it is difficult to digitize the signal processing in the camera.